In magnetic resonance (MR) imaging, tissue contrast is generated by a combination of intrinsic tissue properties such as spin-lattice (T1) and spin-spin (T2) relaxation times, and extrinsic properties such as imaging strategies and settings. Signal intensity in conventional MR images is displayed on an arbitrary scale, and thus is not adequate for direct comparisons.
A major advantage of T1-mapping is absolute quantification of structural changes that is largely independent of imaging parameters, allowing for objective comparisons between examinations. T1 relaxation times depend on the composition of tissues, and while not unique for any specific tissue type, they exhibit characteristic ranges of normal values at a selected magnetic field strength. Changes in tissue composition can either increase or decrease T1 values. For example, increased extracellular fluid is associated with increased T1. Deviation from established ranges can thus be used to quantify the effects of pathological processes. Focal and global T1 changes are reported in a number of parenchymal tissue diseases, such as infarction, fibrosis, steatosis, and systemic diseases such as diabetes mellitus, amyloidosis, sarcoidosis and systemic lupus erythematosus.
T1-mapping may be a sensitive technique for detecting, for example, diffuse fibrosis in heart failure and valvular heart disease. Image acquisition, however, is affected by motion due to aortic and cardiac pulsation and to respiratory movement, which can generate artefacts. Therefore cardiac-gated T1 maps acquired with minimal heart associated movement, in addition to minimal respiratory movement of the diaphragm are preferred for consistency.
One method for performing myocardial T1-mapping is the modified Look Locker inversion recovery (MOLLI) pulse sequence. The MOLLI pulse sequence merges images from three consecutive inversion-recovery (IR) experiments into one data set and has been used to generate single-slice T1 maps of the myocardium. It is generally described in Messroghli D R, Radjenovic A. Kozerke S, Higgins D M, Sivananthan M U, Ridgway J P. Modified Look-Locker inversion recovery (MOLLI) for high-resolution T1 mapping of the heart. Magn Reson Med 2004; 52:141-146 which is incorporated by reference as if fully set forth herein.
The MOLLI technique is triggered by heart beat and attempts to generate a T1 map during a single breath-hold while the heart is not moving. The MOLLI technique does not address high heart rate or irregular heart beat. Moreover, the MOLLI technique involves relatively long recovery epochs, prolonging measurement time. Thus, one perceived shortcoming with the MOLLI technique is the long 17 heart-beat breath-hold required to perform MOLLI. Such a long period may be challenging for many patients who suffer from breathlessness, especially older or obese subjects.
Additionally, currently there are no clinical quantitative magnetic resonance (MR) protocols for the diagnosis of disease in non-moving visceral organs, such as the liver or pancreas, which may be subject to heart associated movement or pulsation. Previously published studies have concluded, for example, that there is no justification for the use of proton nuclear magnetic resonance imaging techniques or the in vivo measurement of hepatic T1 relaxation time. (“MRI of parenchymal liver disease.”, Clin. Radiol. 1987 September; 38 (5): 495-502; Aisen et al., “Detection of liver fibrosis with magnetic cross-relaxation.”, Magn. Reson. Med. 1994 May; 31 (5): 551-6; Alanen et al., “MR and magnetisation transfer imaging in cirrhotic and fatty livers.”, Acta. Radiol., 1998 July; 39 (4): 434-9). For a clinically useful tool, further refinement in MR imaging to assess parenchymal tissue fibrosis has been desired to allow differentiation between non-alcoholic fatty liver disease (NAFLD), which is relatively benign, and non-alcoholic steatohepatitis (NASH), which has a worse prognosis and is more strongly linked to coronary artery disease.
Accordingly, there is a need to address the aforementioned deficiencies and inadequacies.